This invention is directed to the preparation of esters of 5-formyl-3-methyl-2,4-pentadien-1-ol. This bifunctional molecule is an important intermediate in the synthesis of vitamin A particularly by Wittig condensation with an ionyl phosphonium salt (Pommer, Angew Chem., 72, 811 (1960). This diene has previously been prepared by uneconomic procedures requiring multi-step operations (Pommer, Ibid.). The procedure disclosed and claimed herein provides a simple and direct alternative to prepare the bifunctional diene of interest. The instant process employs as starting materials 2-substituted-3-methyl-6-alkoxy-3,6-dihydro-2H pyrans, which are products of the Diels-Alder reaction of 1-alkoxy-1,3-pentadiene and heterodienophiles.
The preparation of 2-substituted-3-methyl-6-alkoxy-3,6-dihydro-2H-pyrans by the Diels-Alder condensation of 1-ethoxy-1,3-pentadienes with ethyl glyoxylate, chloral and formaldehyde has been reported previously in the literature (Shavrygina et al., J. Org. Chem. USSR, 2, 1394 (1966). Other Diels-Alder condensations of 1-alkoxydienes with esters of glyoxylic acid have been reported by Konowal et al. Rocz. Chem., 42, 2045 (1966) and by Makin, Russ Chem. Rev., 38, 237 (1969). An extensive series of articles has been published on the use of 2-substituted-6-alkoxy-3,6-dihydro-2H-pyrans as starting materials for the synthesis of monosaccharides. Among such articles are Chimelewski et al., Rocz Chem., 46, 627 (1972), Zwierzchowska-Nowakowska et al., Rocz Chem., 44, 1587 (1970).
In addition to the above mentioned studies with 1-alkoxy-dienes, Zwierzchowska-Nowakowska et al., Rocz Chem., 48, 1928 (1974), describes the Diels-Alder condensation of 1-acetoxy-1,3-butadiene with butyl glyoxylate to form acetoxy substituted 2H-dihydropyrans.
Hydrolysis of 2-substituted-6-alkoxy-3,6-dihydro-2H-pyrans in the presence of phenylsemicarbazide reagent to afford delta-hydroxy-alpha, beta-unsaturated aldehyde semicarbazones has been described by Shavrygina et al. (supra) and the reaction of 1-methoxy-3,6-dihydro-2H-pyran to afford penta-1,3dien-1-al during the course of steam distillation from phosphoric acid has been described by Woods and Sanders, J. Am. Chem. Soc., 68, 2483 (1946).
The dealcoholation and isomerization of 2-substituted 6-alkoxy-3,6-dihydro-2H-pyrans to form 1-substituted-5-formyl-1,3-pentadienes has been unknown heretofore.
The instant invention provides the art with a novel, efficient and economical method of preparing esters of 1-hydroxy-5-formyl-3-methyl-2,4-pentadiene.